1. Field of the Invention
Embodiments of the present invention generally relate to photomasks used in the semiconductor industry to manufacture integrated circuits and, more specifically, to phase shifting photomasks and methods of their fabrication.
2. Description of the Related Art
In the manufacture of integrated circuits (ICs), or chips, patterns of different layers of the chip are created using reusable photomasks, or reticles. Typically, a set of 15-30 or more reticles is used to construct a chip, and the reticles can be used repeatedly. Herein, the terms “photomask” and “reticle” are used interchangeably. Each photomask lithographically transfers, or prints, the design of a respective chip layer in a photoresist layer disposed on a semiconductor substrate. The precision of the photomask is very important as defects in photomasks may be transferred to the chip, potentially adversely affecting circuit performance.
A conventional photomask generally comprises a light-transparent glass or quartz substrate having a patterned layer of chromium that forms opaque regions of the image to be transferred in the photoresist layer. Photomasks suitable for printing advanced sub-150 nm features utilize interferometric properties of light used by an illumination source of the respective lithographic system and are commonly known in the art as phase shifting photomasks. A phase shifting photomask comprises patterns of regions having alternating optical lengths. Typically, in adjacent regions, a difference in the optical lengths is about a half of a wavelength of the light utilized to expose the photoresist. Such photomasks facilitate transfer of the images with resolution that exceeds optical limitations of the conventional photomasks.
However, despite the considerable effort in the art devoted to development of the phase shifting photomasks, further improvements in the photomasks and methods of fabricating thereof would be desirable.